In recent years, from the viewpoint of environmental protection, the motor vehicle industry has promoted technological innovation, as is seen in a hybrid car that has been in practical use. Besides, an advanced vehicle having a stop-idling function—the engine temporarily makes a stop under a certain condition and automatically restarts—has been already on the market.
Vehicles with a stop-idling function operate on various driving systems: a 14V system and a 42V system. The 14V system employs a single battery with a voltage rating of 12V, whereas the 42V system employs a combined use of a battery with a voltage rating of 36V and a battery with a voltage rating of 12V. Compared to the conventionally used 14V system, the 42V system needs a higher cost. In terms of low-cost production and operation, manufacturers are pursuing the development of the stop-idling function on a vehicle with a single battery.
An electric double layer capacitor is a component essential to the technology above. The electric double layer capacitor is generally used with a conventionally used lead acid battery. Japanese Patent No. 3400319 discloses a method for charging an electric double layer capacitor with the use of braking energy.
Japanese Patent Unexamined Publication No. H02-259276 discloses an improved method for starting the engine. According to the method, a starter is driven by a structure having an electric double layer capacitor connected in series to a lead acid battery. The structure decreases a load on the lead acid battery, increasing the operating life.
According to the conventional engine start device, however, a relay switches between the in-series circuit and the parallel circuit of the battery and the electric double layer capacitor that carry large current needed to drive the starter. The relay switching operation applies a voltage to the starter about two times higher than that applied to the battery, that is, the starter undergoes an amount of change in voltage two times greater than that of the battery.
The battery and the electric double layer capacitor have internal resistance that varies wildly depending on ambient temperature and operating environment, and accordingly, the voltage applied to the starter varies depending on conditions. This degrades a reliable operation in engine start.
Besides, when the electric double layer capacitor is connected in parallel to the battery, the circuit undergoes short-circuit current. This has an ill effect on reliability of the electric double layer capacitor and the relay operations.
When a vehicle that operates on the 14V system (i.e., that employs a battery with a voltage rating of 12V) stops at an intersection and the stop-idling function is activated during the stop, a generator feeds no power since the engine stops. That is, during the stop-idling time, the electric power consumed by vehicle-mounted devices, such as an audio system and a navigation system, is fed from the battery. The battery voltage therefore decreases depending on the length of the stop-idling time and the amount of power consumed by the vehicle-mounted devices.
In a conventional vehicle with no stop-idling function, the generator keeps the battery in a fully charged condition. On the other hand, in a vehicle with stop-idling function, the battery is not always kept in the fully charged condition and the engine sometimes restarts under an unstable condition of the battery voltage.